When using a common tape measure, you have to pull the tape out directly by hand and withdraw the tape by the returning force of the spring, which is very inconvenient. Therefore, Chinese patent No. 94245836, titled “Multifunctional Power-Operated Measure” presents a structural design. It includes a case, a withdrawing reel, a guiding axle, a permanent magnet motor with worm, a withdrawing reel of the worm wheel tape, a clutch mounted between the said worm wheel and the withdrawing reel of the tape, an auto-reset button switch controlling the motions of the tape measure and a battery unit consisting of 4 batteries in series. Though it has many advantages as described, there are still many drawbacks. For instance, too complex in structure, too many useless-functions, inconvenient in manufacture as well as too many measuring devices being crowded in the case to get stuck in the tape measure and to cause the stability decline obviously. Furthermore, there is a major defect in the transmission design of this kind of power-operated tape measure. In fact, it is not practical to realize the withdrawal or sending of the tape merely by the direct or reverse turning of the motor, because during the course of taking out the tape, the tape is often blocked, resulting in the difficulty in swift and smooth sending or withdrawal, which influences the normal operation and measurement. Another example is U.S. Pat. No. 5,471,761, titled “Power-Driven Tape Measure”, which consists of a case, an annular reel, a tape, a motor, a power supply and control switches. The annular reel is pivotally mounted on the fixing shaft of the case, the tape is wound on the annular reel, the hook at one end of the tape is extending out of the case through the outlet of the case, the motor drives the connecting the annular reel and the large wheel for sending the tape on the tape respectively via the gear train, the control circuit consisting of a motor, a power supply unit and a control switch controls the motor for direct and reverse turning, the control switches are engaged with the braking teeth matching the duplex gear disc in the gear train via the link mechanism. This patent is compact in structure and small in size. However, it also realizes the sending or withdrawal of the tape merely by the direct and reverse turning of the motor. Moreover, it requires the cooperation with the braking teeth, resulting in inconvenient operation and the aforesaid drawbacks, e.g., blockage of the tape when being sent out and great friction due to the tape's clinging to the wheel when drawing back. Therefore, further improvement is still required.